1. Field of the Invention
This invention relates to an improved method for upgrading the pozzolanity of fly ash produced from coal-burning power plants and enhancing its value as a replacement for Portland cement in concrete mixes, as well as other uses. More specifically, the invention relates to an economical, high speed method for removing a predetermined amount of carbon particles from coal combustion fly ash in a controlled and predictable manner.
2. Description of the Prior Art
Fly ash, a major by-product of coal combustion, is produced in very large quantities at coal-fired electric utility power plants throughout the world. In the United States alone, the annual volume of fly ash produced by such plants is reported to be in the range of 50 million tons per year. Of this total, it has been reported that only 25-30% of the fly ash is being reused commercially and that 70-75% is being disposed of in landfills. The rapidly diminishing availability of landfill space and escalating cost of land disposal have made it essential to increase the commercial applicability of coal combustion fly ash and/or separate the fly ash into components that have commercial value.
Concrete generally consists of cement, such as Portland cement, water and aggregate. Due to its pozzolanic characteristics, one of the major commercial applications for coal combustion fly ash has been as a substitute for Portland cement in the manufacture of concrete. Some of the advantages attributed to such fly ash as a concrete additive include increased life of the concrete structure, improved flow and pumping characteristics of the concrete, better workability and finishing capability, and decreases in the amount of water and/or lime required in the concrete mix. Most of these improvements are dependent upon increased and controlled air containment in the concrete mix.
It is known that the presence of unburned carbon particles in coal combustion fly ash adversely affects the ability of the fly ash to be used as an additive in concrete. Specifically, the carbon, which is relatively soft and of low strength, does not bond readily with cement and tends to act as a lubricant between particles in a concrete mix. More specifically, the presence of carbon significantly alters the consistency and amount of air entrained in a concrete mix in which fly ash has been used as a substitute for Portland cement. The use of ash having a high carbon content requires greater water addition and the need for incorporating larger quantities of an air entraining agent in the concrete. This is reflected in regulations promulgated by many states which limit the amount of carbon in fly ash used in the manufacture of concrete to less than 5% by weight, and preferably less than 3% by weight, levels significantly below the 6-20% level frequently encountered in most coal combustion fly ash. It is also known that new combustion conditions increasingly being specified in order to minimize NO.sub.x emissions in power plant stack gases result in increased carbon content in the fly ash produced under these new conditions, thereby further restricting the types and amounts of fly ash that can be utilized in concrete.
In order to maximize the commercial application of coal combustion fly ash as a component in concrete manufacture, and to minimize land disposal of same, many methods have been developed to remove carbon particles from the fly ash, thereby minimizing the adverse effects of the carbon on the air entrainment characteristics of the resulting concrete. Such methods have included chemical means to nullify the adverse effect of the carbon component of the fly ash on controlled air entrainment, combustion means to remove carbon particles from the fly ash prior to use, and gravitational, flotational, electrostatic, magnetic, and mechanical means, and combinations thereof, to remove a significant portion of the carbon component of the fly ash before use.
Chemical methods that may be used to nullify the adverse effects of carbon in fly ash without removing the carbon therefrom are disclosed in U.S. Pat. Nos. 4,453,978 and 5,110,362. U.S. Pat. No. 4,453,978 discloses a process for producing a fly ash-containing concrete in which the amount of air contained therein is unaffected by the quantity of carbon present in the fly ash. This approach involves the use of a lipophilic surfactant of a type of sorbitan-higher aliphatic acid ester as an air entrainment agent. Similarly, U.S. Pat. No. 5,110,362 discloses the use of air entrainment agents including water soluble fatty acid salts, abietic acid salts, and ether sulfates added to the active binder in the concrete to nullify the adverse effects of the carbon component on air entrainment in the concrete. Neither of these prior art methods reduce the amount of carbon in the fly ash/carbon mixture prior to incorporation of the mixture into the concrete.
Combustion means for removing the carbon from fly ash by oxidizing all, or a significant portion, of the free carbon present are also known. U.S. Pat. No. 5,390,611 discloses a method and apparatus whereby microwave energy is used to raise the temperature of the carbon component of the fly ash/carbon mixture to the ignition point in an atmosphere containing an excess of air thereby removing the carbon by oxidation. A second known thermal method of reducing the carbon content of fly ash, disclosed in U.S. Pat. Nos. 5,160,539 and 5,399,194, involves introducing the fly ash/carbon mixture into a dry bubbling fluid bed, supplying air thereto, and heating the fluid bed to temperatures in the ranges of 800.degree. F.-1,300.degree. F. and 1,300.degree. F.-1,800.degree. F., respectively. Another known thermal process for removing carbon from fly ash, involving the use of hollow feed screws to tumble the fly ash/carbon mixture in electrically-heated pre-heat and combustion chambers while injecting air and oxygen into the tumbling mass through holes in the hollow feed screws, is disclosed in U.S. Pat. No. 5,390,611. All of these thermal methods, although effective in reducing the amount of carbon present in the fly ash, are energy intensive and involve costly material handling procedures.
Electrostatic means for removing carbon from a fly ash/carbon mixture are disclosed in U.S. Pat. Nos. 4,357,234, 4,514,289, and 4,517,078. In each of these processes, separation is achieved by generating an alternating electric field between electrodes in a manner that causes a centrifugal force to act upon the charged particles of the fly ash/carbon mixture moving between the electrodes such that the lighter, more highly charged carbon particles are separated from the heavier, less charged particles of fly ash. An alternative means of removing carbon from fly ash, disclosed in U.S. Pat. No. 4,556,481, involves moving a fly ash/carbon mixture between a horizontal bottom porous electrode and a horizontal top electrode, maintaining the fly ash/carbon mixture in a fluidized state by means of a gas stream passing upwards through the bottom porous electrode, and maintaining an electric field between the electrodes that is shaped so as to impart a centrifugal force to the charged particles which separates the lighter, more highly charged carbon particles from the fly ash particles.
The use of air classification to separate carbon from a fly ash/carbon mixture into multiple size fractions has been disclosed in Groppo, J. G. et al. "Fly Ash Beneficiation By Air Classification," SME Annual Meeting, March, 1995.
In addition to the aforementioned single-step processes for removing carbon from fly ash, U.S. Pat. No. 4,115,256 discloses a multi-step continuous method for recovering a metal component and a carbonaceous component from a metal/fly ash/carbon mixture. The process involves first passing the metal/fly ash/carbon mixture through a magnetic field to remove the metal component therefrom and, subsequently, passing the metal-free fly ash/carbon mixture through a high tension separator in which the conductive carbon is separated from the dielectric fly ash. A second known multi-step method for separating carbon from a fly ash/carbon mixture containing an iron-bearing component into its constituents, described in U.S. Pat. No. 3,769,054, requires significant concentration of the carbon component prior to its removal from the fly ash/carbon mixture. In the first step of this method, the iron-bearing constituent is separated from the fly ash/carbon mixture by magnetic means. In the second step, the iron-free mixture is subjected to air classification where it is separated into a fine fly ash/low carbon component and a coarse fly ash/high carbon component in which the carbon concentration has been increased to a minimum of 25% by weight. In the third step, the coarse high-carbon component is subjected to screening through a 150 mesh sieve in order to remove the most coarse (+150 mesh) carbon particles. The -150 mesh fraction is then subsequently pelletized, preheated, and sintered to form an improved pozzolanic material.
Multi-step methods for removing carbon from fly ash using flotation means are also disclosed in U.S. Pat. Nos. 4,121,945, 4,426,282, 5,047,145, 5,227,047, and 5,299,692. The method disclosed in U.S. Pat. No. 4,121,945 involves first passing a fly ash/carbon mixture through a 50 mesh sieve to remove only the larger particles of carbon and other agglomerated particles, next adding water to the -50 mesh fly ash/carbon mixture, adding kerosene to the resulting slurry, and then passing the kerosene-containing slurry through a series of flotation cells in which a significant portion of the carbon in the slurry is removed via froth flotation. U.S. Pat. Nos. 4,426,282, 5,047,145, and 5,227,047 describe multi-step methods in which the carbon content of fly ash/carbon mixtures is reduced using wet flotation means which do not involve initial coarse screening.
It has been known to employ wet means, such as froth flotation in processing fly ash. See "A Selective Beneficiation Process for High LOI Fly Ash," Groppo et al., SME Annual Meeting, March, 1996.
Although these prior art separation methods effectively remove carbon from fly ash/carbon mixtures, they are based upon (i) the use of complex, costly equipment, (ii) the need to concentrate the carbon prior to removing it from the mixture, (iii) the use of multi-step processes that involve extensive materials handling, or (iv) the addition of liquid to the mixture prior to separation and the subsequent need to dry the separated components prior to use.
There remains, therefore, a need for a simple, one-step, dry method for reducing the carbon content of fly ash/carbon mixtures without prior concentration of the carbon in the mixture. This reduced carbon/fly ash mixture can then be employed in lieu of Portland cement in concrete. Use of such a method would greatly increase the consumption of coal combustion fly ash as an additive in concrete and significantly reduce the volume of fly ash that must be disposed in landfills.